The invention relates to a method of manufacturing a color display tube comprising, in an evacuated envelope, means to generate a number of electron beams, a display screen having a large number of regions luminescing in different colors, and color selection means having a large number of apertures which associate each electron beam with luminescent regions of one color, and electrodes for forming an electron lens in each aperture.
The invention also relates to a color display tube manufactured according to the method and to a device for carrying out the method.
Such a color display tube of the post-deflection focusing type is disclosed in U.S. Pat. No. 3,398,309. The object of post-focusing is to increase the brightness of the displayed picture by increasing the transmission of the color selection means. In tubes without post-focusing, a very large part, for example 80 to 85%, of the electrons in the beams is intercepted by the shadow mask. By using post-focusing, the apertures in the color selection means can be enlarged, since the beams are focused as they pass through the apertures. As a result, the electron spots on the screen are considerably smaller than the apertures so that in spite of the increased aperture size there is sufficient landing tolerance.
The electron lens which is formed in the apertures of the shadow mask of known tubes is of the unipotential type so that a rather large voltage difference is required between the electrodes which form the lens.
Another post-focusing tube is described in U.S. Pat. No. 2,728,024. In this tube, the electron beams pass successively through two grids each consisting of parallel conductors. The conductors associated with different grids are at right angles to each other. With such an arrangement the electron beams are focused successively by two electron-optical cylinder lenses which are rotated 90.degree. relative to each other. By the action of both lenses together, the electron beams are focused in one direction and defocused in a direction at right angles thereto.
A drawback of this known tube is that it also requires a rather large voltage difference for focusing. In addition, the two grids do not form a mechanical unit so that vibration of the grid wires presents great problems. Furthermore this lens arrangement requires a flat screen.